1. Field of the Invention
The present invention relates to an electrochemical device.
2. Description of the Related Art
In recent years, a lithium ion secondary battery or an electric double layer capacitor is promising as an electrochemical device used as a power source of, for example, electronic equipment or a vehicle.
An organic solvent based nonaqueous electrolyte prepared by dissolving an electrolyte in an organic solvent is used mainly as an nonaqueous electrolyte included in the electrochemical devices referred to above. It should be noted that the organic solvent is combustible. Therefore, in view of the improvement of the safety, various safety devices are mounted to an electrochemical device using an organic solvent based nonaqueous electrolyte, with the result that the volume energy density of the electrochemical device has been caused to be lowered.
Such being the situation, attention is paid to the method of using a nonvolatile room temperature molten salt in place of the volatile organic solvent so as to improve the incombustibility of the nonaqueous electrolyte and thus, to improve the safety of the electrochemical device. The term “room temperature molten salt” noted above denotes a salt consisting of a cation and an anion and capable of assuming a liquid form under room temperature. The room temperature molten salt is featured in having a strong ionic bond and, thus, is nonvolatile and incombustible.
However, in the case of a lithium ion secondary battery using a substance having a lamellar crystal structure, which is represented by a carbonaceous material, as a negative electrode active material, the cationic component of the room temperature molten salt is subjected to a decomposition reaction in the charging stage of the secondary battery, with the result that charge-discharge cycle characteristics of the secondary battery are deteriorated.
Under the circumstances, it is known to the art to suppress the decomposition reaction of the room temperature molten salt by adding additives such as ethylene carbonate (EC) and vinylene carbonate (VC) to the electrolyte used in the electrochemical device, as described in “Takaya Sato, Tatsuya Maruo, Shoko Marukane, and Kentaro Takagi, Journal of Power Sources, 138 (2004) 253-261”. The additive such as EC or VC serves to form a film on the negative electrode in the initial charging stage of the secondary battery so as to inhibit the decomposition reaction of the cationic component of the room temperature molten salt.
However, EC, VC, etc. are incapable of forming a film in an amount conforming to the addition amount. Therefore, if EC, VC, etc. are used in amounts large enough to form a satisfactory film on the negative electrode, the nonaqueous electrolyte is caused to contain large amounts of EC, VC, etc. which are not consumed for forming the film. It should be noted in this connection that EC, VC, etc. are combustible so that it gives rise to the problem that the incombustibility of the nonaqueous electrolyte is impaired.
This is also the case with the electric double layer capacitor. Where a carbonaceous material having a lamellar crystal structure is used as the negative electrode material, a decomposition reaction of the room temperature molten salt is generated in the charging stage of the capacitor so that the charge-discharge cycle characteristics of the capacitor is deteriorated.
Jpn. Pat. Appln. KOKAI Publication No. 2005-149982 is intended to provide an electrolyte, which includes a room temperature molten salt having flame incombustibility and is excellent in the mobility of Li ions. The patent document discloses an electrolyte prepared by dissolving a Li salt in a quaternary ammonium salt consisting of a cation and an anion. It is taught that a lithium secondary battery excellent in the capability of acquiring a large capacity during the stage of the large current discharge can be obtained by using the electrolyte thus prepared. The substituent of the quaternary ammonium salt is an alkyl group, an alkoxy group, or a group having a carbonic acid ester structure or a fatty ester structure.
However, it was impossible to obtain excellent charge-discharge cycle characteristics by using the electrolyte disclosed in the patent document quoted above.